[metadata] XMP extraction & indexation

[drzraf]

The Exif extractor plugin Display the Exif metadata contained inside the JPG files. (Can locate the image on a map if it contains geolocation data)

I see three significant enhancements (distinct Github feature requests?):

• Extract XMP metadata. XMP standardize user-defined metadata about title, creator, license, description, keywords ... and even rating (Xmp.xmp.Rating). This should not be hard, exiv2 pr -pa does this.
• With such a great quantity of user-provided metadata, it would be nice to index some of them and use when searching.
• Then ideally... give users the possibly to map XMP fields to Pydio metadata namespaces. One possible implementation could be an exif definition metadata namespace configured for a specific admin-provided Exif/XMP field in order to make it sound like regular extensible tag.

People working on image/videos/pdf locally, are likely to use their tool-of-choice to set their metadata. Importing and building upon the (de-facto standard) Exif/XMP could be interesting to those who feel the current (neat) metadata system imposes to duplicate their existing file-based metadata.